A majority of compressors for refrigerators, vending machines and showcases have heretofore used dichlorodifluromethane (hereinafter referred to as "R12") as a refrigerant. This R12 is subject to fluorocarbon regulation because of the environmental problem of destruction of an ozone layer, and R134a is being studied as a substituent refrigerant for R12 as disclosed in, for example, Japanese Patent Publication (unexamined) No. 1-271491/1989.
However, the refrigerant R134a does not have good compatibility with a currently used refrigerating machine oil such as a mineral oil and an alkyl benzene oil. This inferior compatibility leads to the problem that imperfect lubrication of a compressor is caused by an insufficient return of the oil to the compressor or by the suction of a refrigerant which is separated from the oil when the compressor is cold started.
Under these circumstances, the inventors of the present invention made studies on polyolester refrigerator oils which are well compatible with R134a refrigerant. However, the use of a polyolester oil in a rotary compressor causes some problems. For example, the fatty acid formed by the thermal hydrolysis of the oil corrodes the sliding members and the corroded sliding members are worn away. Further, the powder formed by the wear exerts an adverse effect on organic materials of the compressor such as magnet wires of the electric motor element to impair the endurance of the compressor.
The inventors of the present invention have made various studies to use a combination of R134a refrigerant with a polyolester refrigerator oil in a rotary compressor and have found that a polyolester oil used in a rotary compressor for lubricating the sliding members can be protected from the hydrolysis caused by the frictional heat buildup of the sliding members can be protected from the hydrolysis caused by the frictional heat buildup of the sliding members by the addition of specific additives to thereby restrain the corrosion of the sliding membes which is caused by the fatty acid generated by the hydrolysis. The present invention has been accomplished on the basis of this finding.